TM 55-1520-240-T2-1 VIBRATION ANALYSIS2-12-1.1 VIBRATION ANALYSIS SYSTEMATIC APPROACH2-1.11.All helicopters have a certain amount of vibration 2.especially during changes in attitude and at highairspeeds and power settings. Excessive vibrationis any vibration that is different from the vibrationnormally sensed in a particular helicopter, any vi-bration that is noticeable in only one pan of thehelicopter, or any vibration that causes discomfortto personnel or damage to cargo carried by thehelicopter.GO TO TROUBLE SYMPTOMTASK 2-1.2.1.STARTThere are two types of vibration, low frequencyand high frequency. Low frequency vibration cyclescan be counted or identified by a beat. A 1/revvibration frequency is 3.75 cycles per second at225 rotor rpm (100% N2). This is slow enough tocount. A 3/rev vibration frequency is 11.25 cyclesper second which is too rapid to count; however,the individual beats are noticeable.4.RECORD AIRCRAFT CONFIGURATIONWas cargo carried either internal or external? Yes  No 2. If internal cargo carried:a. What was weight? _ Ibsb. How was it secured?1 . F l o o r _2 . S t r u c t u r e _3. If external cargo carried:a. What was weight?  Ibsb. What was configuration? 4. What was FAT?  ° C .3.With high frequency vibration, the vibrationto be more of a buzz. Rotor generated vibrationcan usually be observed at 1, 3, and 6 per rev(1350 cycles per minute). 9, 12, and 15 per rev,drive system vibrations tend to be in the higherfrequencies. The Chadwick Helmuth Vibrex balanc-er 177M6A or 177M7A can be used to record vi-brations up to 99,900 cycles per minute (1665Hz). The Chadwick Helmuth Spectrum Analyzer,Model 192A, is capable of recording vibrations upto 900,000 cycles per minute (15,000 Hz). Table2-1.2.2 lists the most common CH-47D excitingfrequencies in Hertz (cycles per see), and cyclesper minute at 225 rotor rpm (100% N2).RECORD VIBRATION CHARACTERISTICS(Circle characteristics)Vibration frequency:a.Vibrtion frequency of 3.75 cycles per second (1/rev) could be counted(low frequency).b. Vibration frequency was too rapid to count but frequency beatwas noticeable (low frequency) (3/rev).c. Vibration frequency was too rapid to count and was or wasn'taccompanied by a buzz or high-pitch tone (high frequency).d. Vibration caused double vision (high frequency).Vibration location:e. Both sides of cockpit.f. Pilots side of cockpit only.g. Copilots side of cockpit only.h. Directional pedals.i. Cabin.Vibration strongest in:j. Cockpit.k. Cabin.Flight phase vibration noticed:l. All the time.m. Only when cyclic changes or large power changes made.Note: If m. circled, identify maneuver.n. Takeoff.o. Transition.p. Climb.q. Autorotationr. Landing.s. Other (describe).The Vibrex system 177M6A or 177M7A shouldbe used to measure low frequency rotor generatedvibration (1, 3, and 6/rev). The Chadwick HelmuthModel 192 Spectrum Analyzer should be used tomeasure drive train vibration and higher frequencyrotor vibration (9, 12, and 15/rev).To identify and locate cause of vibration, recordaircraft configuration and vibration characteristicsas outlined below. Then, determine vibration ex-perienced and go to the task identified.WAS VIBRATION IDENTIFIEDAS LOW FREQUENCY? (A OR BCIRCLED)GO TO TROUBLE SYMPTOMTASK 2-1.2.2.END OF TASK2-2.6Change 5